The NAVSTAR Global Positioning System (GPS) is used to determine exact geographic position (i.e. latitude, longitude, and height above the earth) as well as the exact velocity and time of stationary or moving objects. The navigation receiver calculates position, and time by determining distance to a series of satellites. The navigation receiver calculates velocity by determining doppler frequency shift of the satellite signals.
The NAVSTAR GPS receiver must receive signals generated from the satellite about 11,000 miles away. Each GPS satellite transmits a 6-watt signal. The satellite and receiver, therefore, employs spread spectrum techniques to differentiate the signal from the noise. This is essential since at the antenna the GPS signal is typically about 20 dB below ambient cosmic noise. "Spread spectrum" means that the frequency or instantaneous phase of the signal being transmitted changes as a function of time. Using spread spectrum signal processing techniques, the receiver can track the spread spectrum signal coming from the satellite by estimating a duplicate image of the signal. A precise match of the satellite's spread spectrum signal produces a potential signal processing gain of up to 53 decibels.
Each satellite generates two spread spectrum signals centered around separate frequencies. The L1 channel is centered about 1575.42 MHz, and has course/acquisition-(C/A-) code and precision- (P-) code modulated on it. C/A-code has a 1.023 MHz chipping rate with a band width of about 2 MHz, and P-code has a 10.23 MHz chipping rate with a bandwidth of about 20 MHz. The L2 channel is centered around 1227.6 MHz and only has P-code modulated on it.
For some applications it is desirable to track both L1 and L2. A position can be derived from just the C/A-code or P-code on the L1 band. However, there is ionospheric delay that unpredictably affects the perceived range to the satellite from the receiver. These errors can be corrected by tracking both L1 and L2 and by measuring the difference in the range that is perceived by the receiver on these L-bands. In this manner, the ionospheric errors can be reduced and the navigation solution can be made more accurate by a few meters. In addition, tracking both L bands simultaneously provides more anti-jamming immunity for operation in hostile environments.
The easiest way to track L1 and L2 is with an independent tracking loop for each channel. In all implementations to date, either a single channel is multiplexed between L1 and L2 to reduce hardware requirements or an additional complete hardware channel is provided. This either compromises performance and increases software complexity or increases hardware. In either case, this capability adds significant cost to the receiver.
For some operational scenarios, it may also be desirable to be able to switch between having two independent tracking loops and having one tracking loop with a phase delta between L1 and L2. Two independent loops will be used in hostile environments where there is a good possibility of losing either L1 or L2.
As an example of the problem, consider the situation of navigating above 80.degree. latitude north. Above 80.degree. latitude, the Northern Lights and sun spot activity in the north pole make the ionosphere so active that there is the need for frequent L1 and L2 calculations to maintain GPS system accuracy. Using present systems, performing these frequent L1 and L2 calculations makes it is necessary to add additional hardware channels to the receiver. This increases space and cost for each receiver, and increases processor throughput and software complexity. Computer resources to support the increased number of hardware channels forces the user to make some performance compromises. If a system existed that can track L1 and L2 without the penalty of additional hardware, increased computer throughput and increased software complexity it would have value for the GPS user community.
In known systems, it has been necessary to use two channels to track both L1 and L2. The same P-code information is transmitted on the L1 and L2 bands. However, the receiver sees a phase shift between the two signals because of ionospheric delay. The amount of delay is a function of the level of the ionospheric influence. Therefore, if it were possible to use the fact that this information is related, then it may not be necessary to use two wholly independent channels for tracking L1 and L2.
Accordingly, a need exists in the art for a system and method of tracking both L1 and L2 in a GPS receiver without resorting to two distinct hardware channel receivers or time sharing a single channel.
A further need exists in the art for such a receiver designed in a way so as to minimize power consumption and size.
A further need exists in the art for method and system of providing a GPS receiver which allows for both single and double looped processing of L1 and L2 interchangeably. This allows for selecting between the high jamming immunity of tracking L1 and L2 independently or the processor throughput savings of tracking L1 and L2 together.